Thermal cells are primary electrochemical cells having an anode, a cathode and an electrolyte that is solid and non-conductive at normal temperatures. The cell is activated by providing sufficient heat to melt the electrolyte which thereby becomes conductive. Thermal batteries are made up of a plurality of thermal cells and conventionally also include a heat source, usually an ignitable, exothermically reactive chemical charge or pyrotechnic.
A variety of electrochemical systems are known for use in thermal cells. The electrolytes are generally mixtures of alkali metal halides, most commonly a eutectic mixture of LiCl and KCl melting at about 352.degree.C., although other fusible salt mixtures have been used, such as alkali metal thiocyanates. Suitable active cathode materials that are reduced in the electrochemical cell reaction, often called depolarizers, include phosphates, metal oxides, borates and chromates, the most widely used being calcium chromate or vanadium pentoxide. In many batteries the electrolyte is supported on glass or ceramic fiber tape or felt and the depolarizer is applied as a glaze or paste to a metallic cathode current collector, as by O. G. Bennett and John P. Wooley in U.S. Pat. No. 3,575,714. It is now common practice to mix the electrolyte and depolarizer with a binder in powder form and press the mixture into a wafer, generally referred to as a DEB pellet, as is disclosed, for example, by D. M. Bush in U.S. Pat. No. 3,677,822, S. C. Levy in U.S. Pat. No. 3,425,872 or R. P. Clark and Kenneth R. Grothaus in U.S. Pat. No. 3,527,615.
Calcium is the most widely used anode material, generally in the form of a coating on a nickel or iron current collector, although magnesium is sometimes used and other anodes have been investigated, including solid lithium alloys. Richard E. Panzer in U.S. Pat. No. 3,367,800 discloses the use of a solid lithium anode, in which the cell temperature does not exceed the melting point of the anode. Even small amounts of liquid metal, such as lithium-calcium alloy formed during operation of cells having a calcium anode and a lithium containing electrolyte, result in internal shorting which is a principle mode of failure in thermal batteries, as is pointed by Clark and Grothaus, supra. Various techniques have been used to inhibit liquid alloy formation, while E. M. Klopp and Terrence J. Kurtzweill in U.S. Pat. No. 3,533,844 have used individually sealed cells having a screen barrier adjacent a calcium anode to retain such alloy by capillary action.
Conventional thermal cells, such as the calcium/lithium chloride-potassium chloride/calcium chromate cells, also have disadvantages resulting from self-discharge reactions in which the cell components react chemically, rather than electrochemically, with no electrical power generation. One such disadvantage is that they rapidly deteriorate at operating temperature, even when connected in an open circuit. Another disadvantage is that they have a comparatively narrow operating temperature range of about 100 centigrade degrees and, when overheated, the heat generated by the self-discharge reaction further heats the cell to further accelerate the self-discharge reaction; this type failure is known as thermal runaway. Thermal runaway is an especially severe problem in larger battery sizes because it can be initiated by localized hot spots.
Another problem of great practical significance is the inability to accurately predict the performance of thermal batteries of various size and design as well as the extreme difficulty in obtaining high reproducibility.